* Change order of validation in ValidateCompressedTexSubImage3D
to get the errors in the same order as the dEQP tests are
expecting them.
* ES 3.1: Section 8.7, page 169: If the internal format is ETC2/EAC,
the target must be a GL_TEXTURE_2D_ARRAY.
* ES 3.1: Section 8.7, page 171: For sub textures, ET2/EAC formats
also requires exact size to be validated.

Bug: angleproject:2327

Change-Id: Ib049c70a52ed5683885a73fb06503898a85786d1
Reviewed-on: https://chromium-review.googlesource.com/897726
Commit-Queue: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>